This invention addresses the problem of utilizing CSEM technologies to prospect for reservoired hydrocarbons over large tracts of acreage, particularly where little or no information about potential targets is available.
Controlled source electromagnetic surveying is an established geophysical method for identifying electrically anomalous conductive or resistive bodies in the subsurface. See, for example, Kaufman and Keller, Frequency and transient soundings, Elsevier Science B.V. (1983)). CSEM is typically but not necessarily used to explore subsurface regions under water (Marine CSEM, or “MCSEM”); i.e., CSEM may be used on land as well as in the marine environment. Mineral deposits, hydrocarbon reservoirs, and other geologic bodies, including volcanic, carbonate, and salt features, can have electric resistivity values different from background values (Zhdanov and Keller, The geoelectrical methods in geophysical exploration: Elsevier Science B.V. (1994)). MCSEM surveying can be used to measure these subsurface differences in the marine environment. Most MCSEM surveys are conducted by a moving vessel, towing a submerged electromagnetic signal source, typically a horizontal electric dipole transmitter, over an area where stationary electromagnetic receivers have been located on or near the tow-line, at or near the water bottom. The receivers record electromagnetic signals received, as a function of time.
To the best of the inventors' knowledge, all MCSEM surveys acquired to-date have been conducted in what may be called target-oriented mode. In target-oriented mode, surveys are located, designed, acquired, and analyzed with reference to specific subsurface targets, usually of scientific or economic importance, that have been previously identified using seismic data or other information. Conversely, if MCSEM surveys could be conducted in what may be called reconnaissance (or prospecting) mode, surveys could be located, designed, acquired, and analyzed without reference to specific subsurface targets or any pre-existing information. This would allow large tracts of marine acreage to be surveyed and assessed for the presence or absence of electrical anomalies consistent with reservoired hydrocarbons or other electrically resistive or conductive bodies of economic value. Doing this, however, would require innovative approaches to several aspects of conventional MCSEM surveying. The present invention provides such techniques.
Over the last several years, MCSEM surveying has been increasingly used (in target-oriented mode) to detect, map, and characterize hydrocarbon reservoirs beneath the seafloor. In target-oriented mode, the survey is located, designed, acquired, and analyzed with reference to specific targets, usually of scientific or economic importance, that have been previously identified using seismic data or other information. This is due to two main factors. First, optimal MCSEM signal response is highly dependent on optimizing acquisition parameters to best elucidate the survey target. Second, interpretation of MCSEM data in the absence of other information is notoriously non-unique; see for example, U.S. Pat. No. 6,603,313 to Srnka. Several studies have shown, however, that when data collection is optimized for a particular target, and MCSEM data are integrated with a priori information from seismic or other data regarding the location, depth, size, shape, and reservoir characteristics of that target, MCSEM data can be used to estimate reservoir fluid type. See, for example, Kong, F. N., et al., “Seabed logging: A possible direct hydrocarbon indicator for deepsea prospects using EM energy,” Oil and Gas Journal, 30-38 (May 13, 2002). This has generated considerable interest in the MCSEM field.
In conventional, target-oriented MCSEM surveying, parameters such as target length, width and thickness, target depth below the seafloor, and the surrounding three-dimensional resistivity structure are used to determine optimal transmitter and receiver locations for delineation of that specific target. Modeling and field results show that using optimal transmitter and receiver locations can be critical in imaging subsurface reservoirs, particularly for small, deep, and/or elongate targets, or for those characterized by low electrical contrast with surrounding subsurface bodies. This is because for most targets the maximum MCSEM response is recorded near the target edges where the low-frequency EM energy takes the longest pathway through and around the resistive body.
Similarly in conventional, target-oriented surveying, parameters such as water depth, target depth below the seafloor, and the surrounding three-dimensional resistivity structure, are used to calculate the optimal acquisition frequencies for delineation of the specific target. In MCSEM surveying, EM fields are generated by a transmitter injecting currents of a chosen low-frequency periodic waveform into the earth. Conventional MCSEM alternating-polarity square waveforms are routinely used. They have a broad frequency spectrum (cosine series), but concentrate the energy in one fundamental component. This type of “narrow-band” waveform focuses most of the transmitted energy to the depths that best delineate the target under investigation.
Further in conventional, target-oriented surveying, the MCSEM data are generally analyzed by comparing the measured response to that of 1D, 2D or (preferably) 3D forward, iterative EM models built from a priori information. Alternatively, subsurface resistivities can be determined by normalizing the electromagnetic field data to measured or synthetic background values. Electromagnetic inversion is also a conventional method for interpreting subsurface resistivities from MCSEM data. Full numerical inversion, however, is extremely computationally intensive, and generally benefits from the inclusion of a priori information.
This invention addresses the problem of utilizing MCSEM technologies to prospect for reservoired hydrocarbons over large tracts of marine acreage, particularly where little or no information about potential targets is available. In such reconnaissance mode, surveys can be located, designed, acquired, and analyzed without reference to specific subsurface targets or any pre-existing information.
The possibility of acquiring electromagnetic data in a reconnaissance or large-array mode, rather than in a target-oriented mode, has been previously considered in the published literature. Most of these are not MCSEM examples, including estimating shallow bathymetry using airborne electromagnetic surveys, detecting brine or contamination plumes using airborne and/or ground-based electromagnetic surveys, and exploring for minerals or hydrocarbons using airborne and/or ground-based electromagnetic surveys.
Patent publications WO 01/57555 and WO 02/14906 (inventors, Ellingsrud, et al.) purport to disclose “a method of searching for a hydrocarbon-containing subterranean reservoir.” WO 01/57555, however, seems to provide no specific teachings on how survey design, acquisition, or analysis can be accomplished in anything other than the preferred, target-oriented mode (in which the reservoir's “approximate geometry and location are known”). WO 02/14906 teaches regarding surveying in “an undetermined area” that the “resistivity in the top layers should be mapped” (page 8, lines 10-12). What is needed is a fully enabled method for performing MCSEM in true reconnaissance mode. The present invention provides such a method.